The present invention relates to a hydraulic circuit for a toroidal transmission.
In the field of transmissions, in particular transmissions for motor vehicles, there is a trend toward continuously variable transmissions. Continuously variable transmissions generally allow the internal combustion engine, which is usually arranged upstream in motor vehicles, to operate independently of the respective vehicle speed within an advantageous engine speed range. As a result, the efficiency of the drive train, which is formed by the internal combustion engine and the continuously variable transmission, is improved. Continuously variable transmissions further provide a particularly high level of driving comfort.
Within continuously variable transmissions, so-called toroidal transmissions have a particular importance, namely, in particular, due to their higher torque capacity relative to continuously variable belt drive transmissions (CVTs).
Within toroidal transmissions, the Torotrak™ system is of particular importance (see www.torotrak.com). This transmission does not require a starting clutch on the input side or hydrodynamic torque converters. It is a full toroidal transmission, which is generally constructed in the manner of a countershaft transmission. A variator ensures a continuously variable adjustment of the transmission ratio. The variator has a drive disk and an output disk, which define a toroidal space. Within the toroidal space, three rollers, which are designed to transmit torque from the drive disk to the output disk, are distributed around the periphery. For the adjustment of the transmission ratio, the rollers are spatially adjusted within the toroidal space.
The adjustment of the rollers is carried out via double-acting hydraulic cylinders. With the Torotrak system, the actuator system required for supporting the torque load is also of hydraulic design, for supporting at least one of the disks in the axial direction. Furthermore, two transmission ratio ranges may be set by two clutches. The actuation of the clutches is also carried out via hydraulic actuator systems. Finally, the torque transmission from the drive disk to the rollers and/or from the rollers to the output disk requires a high cooling capacity, which is generally provided by lubricating oil and/or cooling oil. It also has to be ensured that a lubricating film does not rupture in the region of the contact ellipses between the rollers and the disks.
A hydraulic circuit for such a toroidal transmission is known from GB-A-2 369 164.
The hydraulic circuit known from this publication comprises a tandem pump which delivers oil in two separate hydraulic circuits. The one hydraulic circuit is respectively connected to a chamber of the double-acting piston/cylinder arrangements of the respective rollers. The other hydraulic circuit is connected to the other respective chambers.
In this connection, one piston/cylinder unit is configured as a “master” for each of the hydraulic circuits. Variable throttles are configured therein. During normal operation, therefore, the oil flows through the corresponding chambers into respective proportional pressure control valves. During normal operation, said pressure control valves control the pressure in the chambers (and therefore the force exerted by the respective roller actuators). The master piston/cylinder arrangement has an effect on the throttle function thereof only in the end of travel range. In this connection, the piston head throttles the applied volume flow by closing the outlet opening in the cylinder cap. In this manner, the pistons of these piston/cylinder arrangements protect the actuator system from mechanical stops. These “hydraulic stops” at the end of travel of the roller actuator system represent effective end of travel damping. This end of travel damping requires a hydraulic capacity, in contrast to conventional end of travel damping systems. The end of travel damping additionally allows the interaction with the axial pressure against the variator (disk actuator) to be further maintained. The variator which is hydraulically controlled in this manner is technically considered to be reliable.
Control devices for controlling the range clutches may, on the one hand, be connected downstream of the master piston/cylinder arrangements. In this case, a hydraulic-mechanical connection may occur (for example by means of an alternating check valve (“shuttle valve”)). This compares the pressures before the proportional pressure control valves. The respectively higher pressure serves as a source for the contact pressure of the range clutches operated.
On the other hand, it is also possible to divert the pressure for the range clutches directly from the pressures provided by the respective tandem pump.
A second shuttle valve is provided for the hydraulic supply of a disk controller for hydraulically pressing against at least one variator disk in the axial direction (“end load system”). This compares the pressures which are provided by the tandem pump. The higher pressure serves as a source for pressing against the disk.
A lubricating oil circuit is connected to the hydraulic control circuit. The lubricating oil circuit requires a flow pressure for overcoming the hydraulic resistances. In this connection, the hydraulic resistances of an external cooling system and the parallel resistances of the rollers, disks, bearings and gear set have to be overcome.
This known hydraulic circuit is robust relative to mechanical disturbance variables from the drive train. This is achieved by two separate hydraulic circuits with applied volume flows. The respective mechanical hydraulic connection ensures reliable operation. Reliable emergency operation is, therefore, able to be implemented.
Nevertheless, the hydraulic concept created involves hydraulic interaction at a hydraulic capacity level. In this connection, the pressure from the volume flow, which determines the roller actuator system, is directly used for the disk control and clutch control. Furthermore, two pumps (tandem pumps) are required.
A further hydraulic control system is known from DE 698 07 134 T2 (corresponding to EP 0 866 242 B1) for a continuously variable toroidal transmission.
In this known hydraulic circuit, a single pump is provided. In a main pressure line, a first main pressure is regulated by means of a solenoid valve. Furthermore, a secondary pressure is generated from the pump pressure in a secondary pressure line, and more specifically by means of a further solenoid valve. The two pressures are used for the transmission ratio control (roller actuator system). Furthermore, a lubricating oil flow is diverted from the pump pressure which is partially directed via a cooler. The lubricating oil pressure is also regulated.
A respective clutch control pressure is diverted from the main pressure in the main pressure line by means of appropriate solenoid valves, to actuate range clutches of the toroidal transmission.
The axial contact pressure on the variator disks is carried out via a mechanical spring arrangement.
A further hydraulic control concept with a tandem pump is known from DE 195 34 391 A1.